The present invention is concerned with semiconductor memory devices, and, in particular, relates to a NOR flash memory device and method of serially sensing a data bit.
Semiconductor memory devices are data storage apparatuses capable of storing and retrieving. Such devices can be classified generally as random access memories (RAMs) and read only memories (ROMs). The RAM devices are volatile memory devices that lose their data when the power supply is cut off or interrupted, while the ROM devices are nonvolatile memory devices that are configured to retain their data even without power supply. The RAM devices include dynamic RAMs (DRAMs), static RAMs (SRAMs), and so forth, while the ROMs include programmable ROMs (PROMs), erasable PROMs (EPROMs), electrically EPROMs (EEPROMs), flash memories, and so forth.
Flash memory devices are attractive because they consume relatively low power and are flexible and efficient in inputting and outputting information. For these reasons, they are especially applicable to be employed in portable apparatuses such as digital cameras, mobile phones, PDAs, and the like. Flash memory devices can be generally categorized into NAND and NOR types in accordance with the structural feature of the memory cell array. The NAND flash memory devices are relatively simple in structure, so they are advantageous in enlarging the device storage capacity and cheaper than the NOR type devices. The NAND flash memory devices are commonly utilized as data storage components for USB storage apparatuses or MP3 players. Otherwise, the NOR flash memory devices are operable in high speed devices as code storage components, being usually embedded in mobile telephone terminals that require high-rate operation.
A memory cell of the NOR flash memory device is formed by having source and drain regions doped with N+ impurities, between which a channel region is interposed in a P-type semiconductor substrate. The memory cell also includes a floating gate formed by interposing a thin insulation film under 100 Å on the channel region, and a control gate formed by interposing an insulation film on the floating gate. The source, the drain, the control gate, and the substrate, of the memory cell, are connected to bias voltages during programming, erasing, or reading operations.
For instance, in a reading operation, a selected memory cell to be read out is supplied with about 1V to the drain region, 4.5V to the control gate, 0V to the source region, and 0V to the substrate. When the reading operation is carried out in the bias condition, a programmed cell does not flow a current from the drain region to the source region, and an erased cell flows a current from the drain region to the source region through the channel. Here, the programmed cell is referred to as ‘OFF-cell’, while the erased cell is referred to as ‘ON-cell’.
On the other hand, it is preferred that the NOR flash memory device have a larger storage capacity in a smaller area. In implementing the small-area large-capacity NOR flash memory device, it is necessary to enhance the integration density, but there are inherent limits to making the integration density higher under present semiconductor processing technology. Thus, studies for methodologies to overcome the limit on processing technology have been conducted to increase the storage capacity even without raising the integration density. One of the methods is storing multiple bits of data in a single memory cell. A memory cell storing multiple data bits is referred to as a multilevel cell (MLC). For example, a memory cell storing 2-bit data contains 4 state levels such as ‘11’, ‘10’, ‘01’, and ‘00’. The 4 state levels are differentiated by differences in the levels of current that flow through the memory cell during a reading operation.
In order to sense or detect multiple data bits stored in the multilevel cell, a sense amplifier and a data buffer are used, as well known in this art. The sense amplifier finds multiple data bits stored in the multilevel cell by detecting and amplifying a difference between a reference current amount and a current amount flowing through the multilevel cell. The data buffer buffers data detected and output from the sense amplifier. The buffered data is stored in a data latch through a data line.
In general, a NOR flash memory device uses serial and parallel sensing techniques in order to detect multiple data bits stored in a multilevel cell. The serial sensing technique is a way of detecting multiple data bits in sequence by means of a single amplifier, while the parallel sensing scheme is a way of detecting multiple data bits at a time by means of plural amplifiers. As the parallel sensing scheme employs a multiplicity of sense amplifiers, sensing speed is faster but a sensing margin is degraded due to mismatches between different sense amplifiers.
Otherwise, the serial sensing scheme is more advantageous than the parallel sensing scheme in overcoming the problem of mismatches because it uses a single sense amplifier. However, as the serial sensing scheme detects multiple data bits in sequence by means of a single amplifier, a data line may be influenced by a data bit generated from the previous sensing step. Further, the condition of the data line frequently changes from a high level to a low level or from a low level to a high level, resulting in a decrease of sensing margin in accordance with variation of state levels.